The B-H loop of ferrites consists of two areas: one is the DC hysteresis loop and the other corresponds to the dynamic magnetic loss. The former is temperature dependent whereas the latter is temperature independent. The difference in the temperature dependence of these two areas suggests that the physical mechanism for the dynamic magnetic loss is different than that of the DC hysteresis loss. The eddy current loss in ferrite grains is a candidate for the dynamic magnetic loss. The conductivity of the ferrite grain was estimated from the experimental results of the dynamic magnetic loss. Based on the results, it was found that the conductivity is too large for iron oxide. This fact leads to some important suggestions.

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  Magnetic recording of a hard disk drive is negatively affected by a trilemma of noise, thermal fluctuation, and writability, which prevents high recording density. Microwave-assisted magnetic recording (MAMR) is a promising recording method for solving this trilemma. In this study, methods for increasing the AC-field frequency generated from a field generation layer (FGL) and recording performance were investigated using a micromagnetic simulator in consideration of magnetic interactions of an MAMR system. Firstly, we discuss methods for increasing the frequency of the AC-field by changing saturation magnetization (M_s) of the FGL. When the M_s of the FGL decreased, the AC-field amplitude decreased and its frequency increased. When the M_s of the FGL was lower than 1.4 T, the frequency was larger than 20 GHz. Secondly, we discuss the recording performances. As a result, to improve recording performance, high frequency of AC-field (> 20 GHz) is more important than the strength of STO field.

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  The advantages of increasing the writing temperature T_w in heat-assisted magnetic recording (HAMR) are summarized. As T_w increases, the anisotropy constant ratio, which is a new parameter we introduced, can be decreased since the anisotropy constant at the working temperature and the heat-transfer thermal gradient are increased. A relatively thin recording layer (RL) is allowable since the heat-transfer thermal gradient is reduced by increasing the RL thickness because of the adiabatic effect of the RL. Relatively large standard deviations in grain size and anisotropy are permissible since the probability of magnetization reversal is low even for small grains and grains with small anisotropy during the writing period, respectively. A relatively large standard deviation in the Curie temperature is allowable since the heat-transfer thermal gradient is increased. For Fe-Ni-Pt films, the temperature dependence of the anisotropy field is suitable for HAMR when T_w is high. All of the above are advantageous when preparing HAMR media.

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  Many offshore wind-power generators over MW-class are currently in operation in Europe and are now starting to be used in Japan. In order to develop large-scale offshore wind-power plants, we need to reduce the size and weight of power converters, including isolating transformers. A dc-dc converter with a high-frequency transformer is one of the best ways to reduce the size and weight. However, copper loss of the transformer due to proximity effect is a problem because the operating frequency is increased from the commercial frequency to several kHz. In this work, we investigate an interleaved winding arrangement for the high-frequency transformer. We compared the winding resistances of the interleaved and non-interleaved winding arrangements in an experiment using a test amorphous transformer and found that the interleaved winding arrangement inhibits the proximity effect and reduces the winding resistance. We designed an MW-class high-frequency amorphous transformer with the interleaved winding and demonstrated its very high efficiency.

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Two main obstacles to the wider adoption of electric vehicles are short cruising distances and long charging times. We have proposed contactless charging systems for moving electric vehicles utilizing electromagnetic induction. A problem in these systems is high level magnetic field spreading far and wide from feeding and receiving coils, which can affect electronics and human health. In our previous work, we proposed a new feeding coil shape (multipolar coil) that reduced magnetic field at a distance by over 90%. In this paper, to reduce magnetic field from the receiving coil, we newly propose a separated receiving coil and compare it with a conventional spiral receiving coil. Simulations and power transmission experiments revealed that the separated coil greatly reduced the magnetic field far from the coil and achieved high power transmission efficiency of over 80%.

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